1. Field of the Invention
This invention relates to a tracking device and method, and more particularly to an automatic tracking device and method suited for use in the apparatus in which, when reproducing the TV signals or like signals recorded in frequency-modulated form on the concentric circular tracks of a rotary recording medium, the tracking of the reproducing head to the recording track is controlled, while the level of the reproduced signal is monitored.
2. Description of the Related Art
The method of recording TV signals for one field or frame of a still picture on each of concentric, circular tracks in a rotating recording medium has found its use in many recording and reproduction apparatus. FIG. 6 is a schematic plan view illustrating the recording format on the recording medium in the case when such a conventional magnetic recording method is employed. A magnetic recording sheet 2 as the rotary recording medium is rotated about a central axis C by a drive source (not shown). For TV signals of, for example, the NTSC standard, when recording or reproducing them by one field or one frame, the speed of the magnetic sheet 2 is 3,600 or 1,800 rpm. Meanwhile, a plurality of circular tracks 4 are concentrically formed on the sheet 2 when TV signals are recorded from field to field in a certain modulated, for example, frequency-modulated (hereinafter abbreviated to FM) form by a magnetic head (not shown). By the way, for the above-described recording purpose, the correlation of the magnetic head with the track 4 on the sheet 2 is established by positioning the magnetic head on a radial line F in alignment to the track and bringing it into contact with the sheet 2. As to the accessing the track, it is the general practice that one magnetic head is moved along the line F. In order to prevent crosstalk between tracks, a guard band 6 is provided in between any two adjacent tracks 4. In this guard band 6, generally nothing is recorded.
FIG. 7 is a fragmentary cross-sectional view illustrating the relationship between the magnetic sheet 2 and the magnetic head 8. The magnetic head 8 is arranged in alignment with a desired one of the tracks 4 on the magnetic sheet 2. Meanwhile, the magnetic sheet 2 is rotating at a speed of 3,600 rpm or 1,800 rpm. Hence, an air flow is produced to effect a broken contact between them through a very thin air gap 10.
According to such a magnetic recording and reproducing form as has been described above, in each track 4, a respective still picture for one field or frame of TV signal can be recorded or reproduced.
Now, with such an apparatus, particularly when in reproduction, for good reproduced signals to be obtained, not just the requirement that the touching of the magnetic head 8 to the magnetic sheet 2 is sufficient, but the requirement of bringing the magnetic head 8 into accurate alignment with the track 4 on the magnetic sheet 2 must be fulfilled. In general, however, as the magnetic head 8 while being moved along the line F of FIG. 6 comes across a desired one of the tracks 4, an accurate positioning (hereinafter referred to as "tracking") is very difficult to perform, because the mechanical tolerances are one-sided and there is some looseness. Such a tracking error is apt to occur when the reproducing is carried out by using a different apparatus from that used when recording. Even if the mechanical tolerances are cancelled out, for the magnetic sheet 2 having a property of being liable to expand or contract with variation of temperature, such an error will sometimes happen as the temperature is so much changed.
To cope with the above-described problem, a necessity arises that a control be made by moving the magnetic head 8 in either of the directions indicated by arrows A and B to effect an accurate on-track of the magnetic head 8. Such a control is generally known as the auto-tracking.
Such an auto-tracking, in many cases, is performed by positioning the magnetic head 8 in such a position that the reproduced output of the magnetic head 8 becomes maximum.
In that case, the point at which the level of the reproduced RF signal is detected is one in number per revolution of the magnetic sheet and its phase is determined based on the timing of the VD (vertical synchronizing) signal. And, the above-described auto-tracking is operated in a manner which is generally called "mountain climbing control method", as follows: While the magnetic head 8 is being moved a very short distance stepwise for each revolution of the magnetic sheet 2, the levels of the reproduced RF signals produced at the above described respective points on the adjacent two of the revolutions are compared with each other so that the searching is always directed to increase the level of the reproduced RF signal. By taking the point at which this level has once decreased as the maximum, the magnetic head 8 is stopped in this position.
In the mountain climbing control method, however, after the head has once been moved to the target track at a central position B shown in FIG. 8, the auto-tracking is carried out by excursion of the head to the adjacent points A and C to the central one B. In this prior art, therefore, at least four steps of movement of the head: B.fwdarw.C.fwdarw.A.fwdarw.B (on assumption that the maximum is at B) were necessary until the head was stopped.
For this reason, while an inaccurate tracking due to the temperature change or the one-sidedness of the mechanical tolerances did not happen often, although its probability differed with different designs, the use of such a mountain climbing control method for auto-tracking resulted in a large consumption of time, thus giving rise to an alternative problem.